The importance of concrete mix

posted Jan 13, 2018, 5:02 PM by jeffery jim

1. Not many structural engineers (hereafter known as engineers) are involved with design mix as much as they are involved with ultimate limit and service limit design. In most cases, engineers will just adopt the 0.55 water-cement ratio and a typical mix from JKR Building Works specification or recommend the concrete supplier to come out with design mix which matches JKR requirements. There are a few instances, concrete mix should be reviewed when choosing the right cement to match aggregates.

2. Although standard requirement for nominal cover thickness should satisfies code of practice, nevertheless, there should practical approach when considering contingencies for robustness of the design. Usually the exposure of such structures to environment and fire-rating as required by UBBL, substantial considerations should be made depending on the source of aggregate.

3. When designing blast proof structures such as arsenal cache bunkers and boilers, the thickness play a vital role in prolonging the life span in event of explosion for timely evacuation especially for designed compartment(s). In this case, the determination of thickness play a small role (insignificant since its designed for exposure and fire-rating), instead, the water-cement ratio plays an important role where denser concrete will not ruptured or expose the reinforcement to destructive elements aside from proper blast-proofing works.

4. We are exposed to conventional in-situ construction where workability is great even though working at 0.4 cement-water ratio when assisted by gravity. However, that is not the case when involving with pump which needs high workability with the introduction of superplasticizer. Although in certain working condition, the method of construction is not in-situ (IBS), working with gravity has never been great. Wrong design mix for thin wall for IBS shell design is another challenge. In this case, high strength need to be coupled with high workability where the selection of aggregates and binders can be futile as what i have experienced previously with aluminum formwork system for thin shell. Another practice that also affect the concrete dispersion when casting involves the use of bundle bars or some refer it as doubled bar where the surface area of "As" may required larger diameter bars which are not common in the market and doubling the bar is the way to achieved area size for design requirements. Always check for the final design of the bar and work on 5mm aggregate limit for the design mix or readjust the bar bending schedule to allow smooth flowing of concrete.

5. Aggregates and its source provides another set of challenges when making reference to concrete durability in the long run. To most structural engineers, highly alkaline condition is desirable to preserve reinforcement bars from carbonation. That is certainly not the case when using high alkaline cement or certain aggregates which reactive silica content. This increases strength during curing and nominal strength of the structure, it comes with a curse. Highly alkaline materials for reinforced concrete structures can lead to alkali-silica reaction (ASR) which causes much rapid expansion of the concrete by volume or size. If your tiles at home often crack, it is most likely due to ASR. Another consideration or assumption should be made when design mix is the geological properties of the aggregate. Mudstone aggregate is quite porous in comparison to granite which affect the actual water-cement ratio via absorption and wetting condition, carbonation rate and so on which affects the delamination or degradation of the concrete.

6. Another issue which for most inexperience engineers regardless of their age and service period is when conducting compression test for concrete with admixture. Some admixture behaves as retardant for concrete strength growth and 28 days test may not be rendering the right situation of the concrete curing duration. For that reason, resident engineers should request for 4 cubes per sampling rate in order to conduct 7 days, 14 days, 28 days and preferably 60 days before making any decision to take action against the concrete casted in-situ. I have gone through this dilemma (foundation construction) when assisting another consulting firm where statistical analysis for concrete is of no use when batching plant set at site has no previous records and low batching quantity to conduct such analysis and resorting to extracting of concrete core for destructive testing. If you have to, conduct the core test after 60th day which is much more reflective when required standard deviation is more than 1.64.

7. These are a few instances how design mix plays a big role for mainly serviceable limit of concrete in the short run and the long run for reinforced concrete structures.